Cold fluids, such as liquid natural gas (LNG), liquid petroleum gas (LPG), hydrogen, and helium, are maintained at sub-zero temperatures to transport and store such cold fluids as liquids rather than as gases. Liquids require less storage space than gases due to their reduced volume. Cold fluids are typically stored in tanks made with cryogenically compatible materials. Such materials are very expensive thereby substantially increasing the cost of building and maintaining tanks for the storage of cold fluids in the liquid state at sub-zero temperatures. Often the cold fluids are subsequently heated to change the cold fluids from a liquid state to a gaseous state for purposes of use or for transporting through a pipeline.
In one typical situation, natural gas and petroleum gas are transported from port to port as cold fluids by ocean-going carriers or tankers in cryogenic tanks or holds on board the vessel. To maintain natural gas in liquified form at or near atmospheric pressure, the temperature of the natural gas is kept at -270.degree. F. Petroleum gas may also be transported by tanker as liquified petroleum gas by maintaining the temperature of the petroleum gas at -45.degree. F. Upon reaching port, the tanker docks at the pier of an on-shore terminal or facility and connects to articulated piping allowing the transfer of the liquified natural gas or liquified petroleum gas to cryogenic tanks for storing the cold fluids in liquid form. The liquified natural gas or liquified petroleum gas is pumped through the articulated piping to the cryogenic tanks at a pressure sufficient for overcoming pressure losses through the surface piping and for filling the tanks at a pressure slightly greater than atmospheric. The pumps are designed to handle cryogenics and may be low pressure pumps since the off-loading discharge pump pressure is low. The LNG or LPG is subsequently heated to near ambient temperature so that the LNG can be transported on shore by pipeline in gaseous form to another location or in the case of LPG in liquid form. It is preferred that the facility or terminal have the ability to receive at the maximum flow rate of the tanker.
The cryogenic tanks on shore for storing LNG are built at tremendous capital cost. The LNG must be kept refrigerated in these cryogenic tanks at these very low temperatures while stored in the tanks. When the LNG is to be transported by pipeline, it must be heated to vaporize it for transporting through the pipeline. Thus, the facility or terminal must have the capability to heat the LNG. The cost of heating liquified natural gas is approximately $.50 to $.75 per thousand cubic feet of gas. Not only are cryogenic facilities expensive, but the cost of vaporization is also expensive.
The storage of hydrocarbons in naturally occurring conventionally mined, or solution-mined subterranean cavities is well known. Generally, the subterranean cavity is filled entirely with hydrocarbons and an immiscible displacing liquid, such as saturated brine. The hydrocarbons and brine are housed within the cavity in separate phases. When it is desired to introduce additional hydrocarbons into the cavity, a corresponding volume of brine is simultaneously withdrawn. Conversely, when it is desired to withdraw hydrocarbons from the cavity, it is displaced therefrom with a corresponding volume of brine introduced simultaneously into the cavity. Such on-shore storage systems and methods are generally accepted.
Salt storage caverns have most often been used as the subterranean cavity to store hydrocarbons. A cavern is formed in a salt dome or strata by leaching. It is commonplace to drill into the soluble salt formation, suspend a wash pipe and concentric pipe strings into the cavity, and produce brine by solution mining, well known in the art. Dual fluid passages are provided into the cavern in the concentric pipe strings so that fluid handling means at the surface provide a capability for brine to be pumped into and out of the lower area of the cavern and the hydrocarbons can be taken from the upper area. Hydrocarbons are added to storage by pumping the hydrocarbons into the cavern under sufficient pressure to displace the brine therein back to the surface. The displaced brine is maintained at ground level in a brine pit or reservoir, and then is returned to the salt cavern to replenish the volume of brine as hydrocarbons are retrieved. Precautions are taken to assure that the brine in and out of the cavern is always salt-saturated to avoid enlarging the brine cavern. In a typical downhole salt cavern, the hydrocarbons stored in the cavern are removed from the cavern by pumping the hydrocarbons to the surface through the annulus formed by the concentric pipe strings.
It is known to store natural gas in a subterranean salt cavern in the gaseous state. The natural gas is compressed into the subterranean cavern under pressure and at a temperature only slightly different from the temperature of the brine disposed within the cavern. Since there is very little temperature differential between the natural gas and brine, there is only minor counterflow heat exchange between the natural gas and brine as the brine is displaced from the cavern. The temperature of the natural gas is substantially steady-state. It is desirable that the temperature of the natural gas not be substantially reduced since it is desirable that the temperature of the gas not be too low upon removal from the cavern. The natural gas may be under sufficient pressure that upon opening a valve, the natural gas flows out of the cavern under its own expansion.
Natural gas is not stored in a subterranean cavern as a liquid since it must be stored at sub-zero temperatures to maintain the natural gas in liquid form. Storage of the natural gas at such extremely low temperatures would adversely affect the structural integrity of the cavern walls and may cause the cavern walls to fracture. Also, off-loading the liquified natural gas at sub-zero temperatures requires special equipment. Further, if the temperature of the natural gas in the cavern is too low, the brine in the cavern would freeze, thereby plugging up the flow paths through the concentric pipe strings extending into the cavern.
It is known to store liquified petroleum gas in salt caverns in a liquid state. The LPG is maintained at a temperature between 80.degree. and 90.degree. F. and under sufficient pressure within the cavern to keep the LPG in liquid form. Although the LPG is typically shipped at temperatures of-45.degree. F., the LPG is heated before it is stored in the subterranean cavern. Since it is desirable to unload the LPG from the tankers at a very high off-load rate, it is expensive to preheat the LPG at that same very high rate. Further, the LPG is at such a low temperature upon offloading from the tanker that these low temperatures require specialized equipment.
U.S. Pat. No. 2,869,328 discloses introducing hydrocarbons into an underground storage cavity with the brine in the cavity being displaced up through a pipe string. As the brine flows up through the pipe string in countercurrent heat exchange relationship with the incoming cool hydrocarbon, the temperature and pressure of the brine are gradually reduced as the brine approaches the surface causing the precipitation of salt. To prevent the build-up of salt within the pipe string so as not to clog the pipe string, a supply conduit is passed down the pipe string for injecting fresh diluting water into the lower end of the pipe string to dilute the brine and prevent precipitation of the salt within the pipe string. If an unusual deposit of salt were to occur within the pipe string, the supply conduit may be used to flush the pipe string with fresh water and remove the salt deposit.